1. Technical Field
The present invention relates to a method for measuring core portion noncircularity of an optical fiber base material. More particularly, the present invention relates to a method for measuring core portion noncircularity of an optical fiber base material which is used as a raw material for manufacturing an optical fiber.
2. Related Art
In recent years, polarization mode dispersion (hereinafter referred to as PMD) is listed as one of the factors that make it difficult to realize optical communication for a longer distance and at a higher speed. Here, PMD is a phenomenon where two modes that induce polarizations perpendicular to each other are propagated at slightly different speeds within an optical fiber and the width of an input pulse therefore increases as the pulse is propagated in the optical fiber.
When optical communication is significantly affected by the PMD, different pulses of signal light partly overlap each other. In this case, a receiving section can not distinguish the pulses. When higher-speed communication is performed by reducing the pulse width, the PMD may prevent the communication from being performed with a target pulse width. Furthermore, the long-distance communication is exposed to a higher risk, due to the PMD, that the pulses of the signal overlap each other to cause a communication error.
A typical single-mode optical fiber includes a core portion and a clad portion, where the core portion has a higher refractive index than a portion surrounding the core portion and the clad portion covers the core portion. Here, it is mainly the core portion where light is propagated. When the core portion is perfectly circular, the two modes that induce polarizations perpendicular to each other are degenerated and can not be distinguished. When the core portion is not perfectly circular or the core portion loses symmetry because the core portion is deformed during the manufacturing process of the optical fiber or optical fiber cable, the two modes are propagated at different speeds from each other within the optical fiber, thereby causing the PMD.
One of the methods to control the PMD is to measure and control the non-circularity of the core portion of the optical fiber base material. Here, Unexamined Japanese Patent Application Publication No. 2003-042894 discloses the following method for measuring the non-circularity of the core portion. The optical fiber base material is immersed in a liquid filled with a matching oil. While the optical fiber base material is rotated, parallel light is irradiated to the lateral surface of the optical fiber base material. Part of the irradiated light transmits through the optical fiber base material, and is received and imaged. Based on the brightness distribution of the image, the outer diameter of the core portion is measured. The values of the outer diameter of the core portion which are sequentially measured along the perimeter are used to calculate the noncircularity of the core portion.
To control the PMD by controlling the non-circularity of the core portion of the optical fiber base material, it is necessary to predict the PMD which may be caused in an optical fiber that is manufactured from the optical fiber base material. This prediction requires measurement of the non-circularity of the core portion to be performed at fairly small intervals. When the non-circularity of the core portion of the optical fiber is desired to be known at an interval of 1 km, the non-circularity of the core portion of the optical fiber base material with an outer diameter of 80 mm needs to be measured at an interval of approximately 2.4 mm, and the non-circularity of a core base material with a core outer diameter of 18 mm needs to be measured at an interval of approximately 0.2 mm.
The above-described unexamined Japanese Patent Application Publication discloses the method for measuring the outer diameter of the core portion of the optical fiber base material while the optical fiber base material is being rotated. After the optical fiber base material is completely rotated once for the measurement of the outer diameter of the core portion, the base material needs to be moved and held so that the measurement is performed at the next measurement position. Thus, a certain time is required for such movement of the optical fiber base material. Even though the measuring process is automated and the movement of the optical fiber base material from one measurement position to the next measurement position is completed in one second, the movement between the measurement positions alone takes no less than 16 minutes to perform the measuring process on an optical fiber base material having a length of 1000 mm at an interval of 1 mm. When the measuring process is performed at an interval of 0.2 mm on a core base material having a length of 500 mm, the movement between the measurement positions takes no less than 40 minutes. Here, the total time required to measure the outer diameter of the core portion further includes the time for the rotation and the time for the calculation in addition to the time for the movement. As a result, an extremely long time is consumed for the measuring process of the outer diameter of the core portion.
To address this problem, the optical fiber base material may be rotated and moved at a higher speed. This, however, may cause leakage of the matching oil from the inserting section through which the optical fiber base material is inserted even though the vessel for the measuring section which is filled with the matching oil is sealed with the use of an elastic packing material.